1. Field of the Invention
The present invention relates to an electronic throttle control unit for controlling an automotive engine for example, and more particularly, to a new technique in which accuracy in detecting a throttle opening can be improved by using an inexpensive AD converter with a low resolution.
2. Description of the Related Art
In general, in an electronic throttle control unit for an engine, the degree of opening of an electronic throttle is controlled in a feedback manner so as to coincide with a target throttle opening that is appropriately calculated in accordance with the operating condition of a vehicle on which the unit is installed (see, for instance, a first patent document: Japanese patent application laid-open No. H10-222205).
To this end, the electronic throttle control unit (ECU) serves to AD convert an output voltage of a throttle opening sensor, calculates a target throttle opening by using the AD converted value of the throttle opening (detected value), and controls the electronic throttle in a feedback manner.
In addition, particularly during idling operation, it is necessary to control the amount of air (amount of intake air) sucked into the engine in an accurate manner so as to maintain relatively low idling speed, and hence throttle control with high reliability is required.
In order to control the amount of intake air sucked into the engine in an accurate manner, it is necessary to control the electronic throttle with high accuracy, and to this end, it is also necessary to detect an output voltage value of the throttle opening sensor with a high degree of precision.
Accordingly, there has been proposed an electronic throttle control unit in which to detect a throttle opening sensor voltage in an idling speed range with a high degree of precision, an n× (n times) amplifier is provided in the control unit, and two voltages, i.e., a throttle opening sensor voltage (throttle opening voltage) output from an actuator of an electronic throttle and a voltage value obtained by amplifying the throttle opening voltage with the n× amplifier, are read in so that these two voltage values are alternatively switched from one to another in accordance with the operating range of the engine (see, for instance, a second patent document: Japanese patent application laid-open No. H6-101550).
Also, there has been proposed an electronic throttle control unit in which a throttle opening voltage is converted into a plurality of voltages with offsets, which are then read out by a plurality of AD converters or single AD converter, whereby a throttle opening voltage is detected with high accuracy based on the results of AD conversion of the respective voltage values (see, for instance, a third patent document: Japanese patent application laid-open No. 2003-28001).
Further, in the third patent document, there is also proposed a device in which the plurality of voltages with offsets converted through a transistor switch are read by the single, AD converter.
In the known electronic throttle control units for an engine, there is the following problem. That is, in the case of the electronic throttle control units described in the second and third patent documents, for instance, there is a possibility that errors in the detected values might be caused due to a difference or variation in accuracy between the plurality of AD converters, thus exerting adverse effects on throttle control.
In addition, in case where the plurality of voltages with offsets converted through the transistor switch are read by the single AD converter, there exists another problem. That is, there is a possibility that errors in the detected values might be generated due to the internal resistance of the transistor switch, thus causing adverse influences on throttle control.
Further, in case where an AD converter of a high resolution is used so as to detect the throttle opening voltage with a high degree of precision, there is also a further problem of inviting an increase in the cost of the entire unit.